This application is a 371 of PCT/JP98/05462 Dec. 3, 1998.
This invention relates to a cellulose dispersion in which a cellulose is highly dispersed in a dispersing medium and which is high in transparency; to a spherical particulate or spherical composite particulate which is low in aggregable properties and large in hardness; and to a process for producing the same.
A cellulose which is a natural polymer or a particulate whose starting material is a cellulose and dispersions of them have been widely used as an additive agent for drug-molding; a food additive; a cosmetics additive; a binder for various molding purposes; a filter aid; a support for sustained release drug; and the like.
A dispersion in which a cellulose is the starting material and a cellulose particulate are, in all cases, used as a composite system in which it is mixed with and dispersed in other functional materials (drugs, oily agents, perfumes and the like). Therefore, they are required to have a function for highly dispersing the functional materials and allowing them to stably exist in the composite system (referred to hereinafter as the dispersion stability) is required.
However, the conventional cellulose dispersion has such a problem that the cellulose aggregates in an acidic area of pHxe2x89xa63 and it does not function as an additive in acidic products. Moreover, a water-soluble polymer containing a water-soluble cellulose derivative (hydroxyethylcellulose, carboxymethyl cellulose or the like), though it is not a cellulose, has a high dispersion stability in a composite system and is utilized as a thickening agent for paints, cosmetics, cataplasms for drug or the like. These are usually used in the form of an aqueous solution of about 0.5 to 3% by weight; however, for example, it is difficult for a water-soluble cellulose derivative for a water-based paint to always keep the required good balance between viscosity and thixotropic properties (a phenomenon that the viscosity is high at a low shear rate and a viscosity reduction takes place at a high shear rate), and those having a such a viscosity that they are spreadable are low in thixotropic properties, and have problems such as run of liquid being caused and the like. In addition, the sticky feel and spinnability characteristic of a water-soluble polymer are often disliked in some use fields and problems have remained.
Moreover, it has long been desired to substitute for the water-soluble polymer a cellulose dispersion which is a natural material and has a thickening action and a dispersing action for the utilization as a coating agent such as a paint, a cosmetic or the like; however, the conventional cellulose dispersion lacks the smoothness and transparency of coating surface and the improvement of these properties has been required. That is to say, there has been desired a dispersion in which the conventional cellulose particulates are much more highly dispersed.
Furthermore, the water-soluble polymers are also employed in uses requiring the transparency of solution as in cosmetic emulsions and gel-like hair-dressing agent. The conventional cellulose dispersion is a uniformly white-turbid, opaque suspension and has been unable to be utilized at all in this field. In addition, even when a coloring material such as a dye, a pigment or the like is incorporated into the existing cellulose dispersion, there have been problems such as the color developability characteristic of the coloring material being unable to be secured and the like.
Accordingly, in order to impart to the cellulose dispersion a further function such as dispersibility, smoothness, transparency or the like to make it applicable to a wider industrial field, it has been desired to impart thereto so high a dispersion-stability as to be comparable to that of the water-soluble polymer in the complex system and further desired to finely divide the same and impart transparency thereto.
The conventional cellulose dispersions used as various additives is generally composed of a dispersing medium (usually water) and particulates obtained by a method for producing xe2x80x9cmicrocrystalline cellulose (MCC)xe2x80x9d and xe2x80x9cmicrofibrilated cellulose (MFC)xe2x80x9d.
As the prior art concerning the cellulose dispersions, JP-A-3-163,135 discloses a dispersion of a finely divided cellulose material. JP-A-3-163,135 discloses a technique of producing, by a depolymerization treatment and a subsequent wet grinding treatment, a suspension containing at least 2% by weight of an atomized cellulosic material in which the particle diameters whose integrated volume is 50% in the particle diameters distributions with relation to a volume are 0.3 to 6 xcexcm and the integrated volume proportion of particles of not more than 3 xcexcm in diameter is not less than 25%. There is a statement to the effect that as the starting cellulose material used for the production of the above dispersion, a regenerated cellulose material can also be used, but there are no working examples, and according to the above method, there are not obtained those that the fraction of cellulose II type crystal component is not more than 0.4 even when said starting material is used. Moreover, it is stated that a degree of polymerization of not more than 300 is preferred; however, substantially, it is considerably higher than the average degree of polymerization (DP) of the cellulose of the present invention. In addition, as shown in Comparative Example 2 of the present specification, no dispersion having transparency is obtained.
Furthermore, JP-A-3-163,135 states that by finely dividing the cellulose material in the cellulose dispersion, the development of thixotropic properties, water-retention characteristics, viscosity, dispersion stability and the like are improved, and simultaneously therewith, when it is used as, for example, a food additive, the rough feel, foreign matter feel and powdery feel of the food can be diminished and an improvement of sense of touch and an increase of the addition amount can be expected. The same publication points out simultaneously the limit of degree of finely dividing a group of cellulose type particulate additives, the deterioration of dispersion stability by too finely dividing them, the lower limit value (2% by weight) of the particulate concentration in the suspension at which the dispersion stabilization effect is exhibited, and the like.
JP-A-56-100,801 discloses a microfibrilated cellulose. This fibrilliform cellulose is ground in the form of fibers, and there are obtained those having a long diameter (L) of about 100 xcexcm though and having a short diameter (D) of not more than 1 xcexcm (L/D is large and the anisotropy is high). When a dispersion is intended to be prepared from such a microfibrilated cellulose, the dispersion of the present invention low in crystallinity, low in degree of polymerization and high in transparency which comprises a cellulose II type crystal as the main component is by no means obtained as pointed out in relation to JP-A-3-163,135.
JP-A-9-508,658 discloses a technique of preparing a liquid crystalline cellulose film from a colloidal dispersion composed of particles having a particle diameter of several to several hundreds nm by hydrolyzing natural cellulose particulates in concentrated sulfuric acid at a high temperature (60 to 70xc2x0 C.). The cellulose crystallite obtained thereby has been substituted by a charge group to a derivative and aggregation and association are inhibited owing to this charge repulsion, and it has neither thickening effect nor dispersion-stabilizing effect of the cellulose additive as aimed at by the present invention and is different in industrial applicability. Specifically, there are stated application examples in which the optical characteristics of a film obtained from a liquid crystalline, colloidal dispersion are utilized. Since in the technique of the above-mentioned publication, sulfuric acid hydrolysis is effected under the conditions that the cellulose crystallite is not dissolved, no one that the fraction of the cellulose I type crystal component is not more than 0.1 is obtained and the surface of the cellulose particle is converted into a derivative in the course of the hydrolysis, so that it is different from the dispersion of the present invention.
In addition, SU 1,178,763 A discloses a method for the hydrolysis of a cellulose, mainly cotton, which comprises the first stage of dissolving a cellulose in 65 to 75% sulfuric acid (aqueous solution) and reprecipitating the cellulose from that solution under the specific conditions to recover the cellulose in a high yield and the second stage including enzyme hydrolysis of the cellulose. This method is one comprising dissolution in an acid, reprecipitation, filtration, washing, drying and subsequent hydrolysis with a cellulase, and hence different from the present invention. In particular, in the cellulase hydrolysis method, the hydrolyzed cellulose is completely depolymerized into glucose through a water-soluble material such as cellobiose or the like. On the other hand, the present invention includes a dispersion composed of a water-insoluble cellulose and a method for producing the same.
Moreover, JP-A-54-160,801 discloses a method comprising recovering a cellulose and producing glucose from the cellulose recovered. This publication describes a method for obtaining glucose which comprises removing a hemicellulose from a cellulose-containing material such as corn cane, bagasse or the like with an about several weight % dilute acid under pressure, dissolving the residue (lignocellulose) in a concentrated acid, subsequently diluting the solution with a selected solvent to reprecipitate the same, recovering the cellulose in a high yield and further hydrolyzing the cellulose thus obtained with a cellulase, a dilute acid or a combination of a cellulase with a dilute acid. The cellulose dispersion of the present invention has not been obtained.
As mentioned above, such a cellulose dispersion that when added in a small amount, it develops a shaping effect such as thickening property, stability of dispersion, emulsion or the like, structure stability, coating film-formability or the like and further that it can be used in a filed in which transparency is required has never been obtained.
Moreover, a cellulosic particulate can be obtained from the cellulosic dispersion by a spray drying method and such a cellulosic particulate has characteristics such as safety resulting from natural material, hygroscopicity and the like, so that various applications in which the characteristics are utilized are expected. However, the conventional cellulosic particulates have such problems as shown below. That is, (1) those having a high dispersion stability in a composite system and a high mechanical strength (for example, hardness) have a relatively large particle diameter and is poor true sphericity. Moreover, (2) those having a high true sphericity is poor in dispersion stability and low in mechanical strength.
For example, such cases that particulates poor in true sphericity, when added to a cosmetic foundation, cannot sufficiently inhibit the rough feel and lackluster feel nor sufficiently secure the smoothness and rolling properties (spreadability) of a coating surface, and the like are caused. Therefore, the particulates are little used in these applications. Even when they are used in a coating solution for print, they are confronted with the same problem. In addition, the particulates poor in strength have such problems that when a container is filled under pressure with a product to which they have been added or the product is coated by a strong pressure, the fracture or collapse of particles is caused, whereby such changes of product performance as deterioration of rolling properties, developing of a rough feel, reduction of volume and the like are caused. It is added that the particle diameter at which human being does not feel the material rough is said to be about 20 xcexcm or less.
As the spherical particulates, there are known particulates made of a synthetic resin such as polymethyl methacrylate, nylon or the like; however, there have been desired cellulose spherical particulates which can be applied to wide uses as an additive and have no problems mentioned above.
As the cellulosic particulates which have heretofore been known, there are those formed by drying a dispersion of MCC or MFC by a spray drying method, those obtained by subjecting a cellulose solution such as a viscose solution or the like to phase separation in the form of liquid drops by an emulsification method and then to solidification with a coagulating agent. The latter is easier than the former for obtaining spherical particulates. A typical, known technique as to cellulose particulates is explained below.
JP-A-3-111,426 discloses a method for producing spherical cellulose particles. It is a method comprising pretreating the starting cellulose by acid hydrolysis, alkali oxidative destruction, enzyme decomposition, steam explosive disintegration or vapor cooking and thereafter subjecting the same to a step of preparing a suspension, a step of wet grinding and a step of drying by a spray drying method to produce particles. It is stated that in this method, a suspension of finely divided cellulose particles having an average particle diameter of 1 to 2 xcexcm obtained by wet-grinding is dried by a spraying method to obtain spherical particles having an average particle diameter of about 3 to 12 xcexcm. According to the knowledge of the present inventors, the above method is basically a method in which the base technique of MCC is combined with the wet grinding and a drying method under the specific conditions, and the spherical particles obtained are highly crystalline. The specification of the above publication has such a statement that a regenerated cellulose can be used as the starting material, but has no working examples. In the above method, even when the regenerated cellulose is used as the starting material, the crystalline state of the cellulose used is maintained, and there is obtained no product having a fraction of a cellulose II type crystal component of not more than 0.4. Moreover, as shown in Comparative Example 13 of the present specification, even when finely divided MCC in the form of a bar is dried/granulated by a spraying method, the true sphericity does not become so high. In addition, the true sphericity has not been quantified. In these respects, it is different from the present invention.
JP-A-5-200,268 discloses a method for producing spherical cellulose particulates having an average particle diameter of not more than 15 xcexcm. The same publication is a technique of producing the above-mentioned spherical cellulose by mixing a viscose emulsion formed by adding a viscose solution and a surface active agent to a water-insoluble dispersing medium with a viscose-coagulation emulsion formed by adding a viscose coagulation to a water-insoluble dispersing medium and subjecting the mixture to reaction.
The viscose liquid used is a dope xe2x80x9cfor conventional rayonxe2x80x9d obtained from cotton linter or pulp. From this statement, the average degree of polymerization of the particles obtained is at least about 300 and different from that of the cellulose of the present invention. Moreover, it is a production method in which a cellulose converted into a derivative is subjected to regeneration and solidification using a surface active agent and a water-containing coagulation such as aqueous sulfuric acid or the like, and therefore, the particulates obtained include voids based on the phase separation method in their interiors and a tendency to become porous is unavoidable. Moreover, substantially no similar prior art references including JP-A-5-200,268 elucidate a specific drying method for obtaining dry particulates from a suspension of these particulates. According to the knowledge of the present inventors, in order to dry porous particulates without aggregating them while the true sphere is maintained, there is a means such as vacuum drying at a low temperature, freeze drying or the like; however, this is very difficult. It is also difficult to purify the cellulose when it is finely divided. That is, guided by the disclosure of JP-A-5-200268, it is very difficult to produce dry particulates having a small porosity and low aggregable properties and maintaining the original spherical shape. Such dry, porous particulates easily undergo collapse, crushing or the like upon strongly crushing or compressing under high pressure. In this respect, they are different from the cellulose particulates of the present invention which have substantially no porosity and a low degree of polymerization and are spherical and large in strength.
Moreover, Preprint of Lecture at the 5th Annual Meeting of Cellulose Society, p. 92 (1998) states a report concerning the preparation of porous cellulose beads having particle sizes of 5 to 10 xcexcm. The said technique is a technique comprising dispersing a viscose in an alkali solution of a water-soluble polysaccharide, adding sulfuric acid to this dispersion to conduct the coagulation-regeneration of the cellulose and the hydrolysis of the polysaccharide. It reports that because of the porosity, the water-absorbability and oil-absorbability xe2x80x9cshowed a high value as compared with fibrous cellulose and microcrystalline cellulosexe2x80x9d. There is a high possibility that the cellulose beads contain a polysaccharide from the production method described in the above preprint, and the function depends upon the porosity thereof. This technique is basically a similar technique to JP-A-5-200,268, and includes the same problems as previously pointed out.
JP-A-8-283,457 discloses a spherical cellulose which is composed of a regenerated cellulose having a crystallinity of not more than 5% and which has a true sphericity of 0.8 to 1.0 and an average particle diameter of 20 to 2,000 xcexcm and a method for producing the same. The purpose of this invention is to provide a spherical cellulose which has a low crystallinity and is easily subjected to addition reaction and cross-linking reaction, and the utilization field thereof includes a medical substrate, an adsorber, a filler for chromatography and the like. In the method for producing this spherical particle, cellulose is dissolved in a dimethylacetamide/lithium chloride or N-methylpyrrolidone/lithium chloride solvent and thereafter dropped into an alcohol to be coagulated in the form of spheres (dropping method) or the above solution is added to a mixture of a polyhydric alcohol group-containing surface active agent and a dispersing medium and stirred/emulsified to be subjected to dispersive coagulation (dispersive coagulation method).
This spherical particulate has a large particle diameter and is different from the present invention. Moreover, the above publication has a statement to the effect that an organic medium dope of a cellulose is used for avoiding the phenomenon of conversion of the cellulose into one having a low degree of polymerization that the cellulose dope has, and it is clearly different from the present particulate having a low degree of polymerization.
The invention of JP-A-8-283,457 is also a particulate obtained from a cellulose dope and has the above-mentioned problem.
Thus, there have not still been provided cellulose particulates which have a particle diameter of not more than 20 xcexcm and are in the form of a true sphere, difficult to aggregate and high in particle strength.
An object of the present invention is to provide a cellulose dispersion which has a high dispersion stability in a level comparable to a water-soluble polymer in a composite system and has a transparency and which is applicable to a wider industrial field as well as a composite cellulose dispersion and a method for producing them.
Another object of the present invention is to provide a spherical cellulose particulate obtained from the above cellulose dispersion as well as a cellulose composite particulate and a process for producing them.
That is to say, the present invention provides a highly functional cellulose dispersion which, as compared with the existing cellulose dispersion, has a very high dispersion stability for suspension, dispersion, emulsion or the like, and can develop excellent thickening properties, shape retention-imparting properties, tissue-imparting properties, cloudy resistance, sizing properties, coating properties, structure stability, gelation properties, surface activity and the like, and which has a high dispersibility (transparency) comparable to water-soluble polymer solutions, and provides a process for producing the same.
Further, the present invention provides a spherical cellulose particulate which is small in particle diameter, difficult to aggregate and high in particle strength as well as a cellulose composite particulate in which the above particulate has been compounded with a functional additive and a process for producing the same.
The present inventors have found that a cellulose dispersion having a specific degree of polymerization and crystallinity and having a peculiar dispersion structure is dispersed in a dispersion so highly as to be comparable to water-soluble polymers and is consequently very much higher in transparency than the conventional cellulose dispersion and also found that particulates obtained from the above dispersion are in the form of spheres and difficult to aggregate and high in particle strength, and have accomplished the present invention.
That is to say, the present invention includes:
(1) a cellulose dispersion which is a dispersion comprising a dispersing medium and a cellulose having a fraction of cellulose I type crystal component of not more than 0.1 and a fraction of cellulose II type crystal component of not more than 0.4 and in which the average particle diameter of the constitutive cellulose is not more than 5 xcexcm;
(2) the cellulose dispersion according to (1) above, wherein the average degree of polymerization (DP) is not more than 100 and when the cellulose concentration of the cellulose dispersion is adjusted to 0.05% by weight, the transmittance of the cellulose dispersion to a visible light of a wavelength of 660 nm is not less than 40%;
(3) the cellulose dispersion according to (1) or (2) above, wherein the average particle diameter of the constitutive cellulose is not more than 2 xcexcm;
(4) the cellulose dispersion according to (1), (2) or (3) above, wherein when the cellulose concentration is adjusted to 0.05% by weight, the transmittance of the dispersion to a visible light of a wavelength of 660 nm is not less than 80%;
(5) the cellulose dispersion according to (1), (2) or (3) above, wherein the dispersing medium is water and/or an organic solvent;
(6) the cellulose dispersion according to (4) above, wherein the organic solvent is a water-soluble alcohol;
(7) a composite cellulose dispersion which comprises the cellulose dispersion according to (1), (2), (3) or (4) above and a functional additive compatible with the dispersing medium, wherein when the cellulose concentration in the composite cellulose dispersion is adjusted to 0.05% by weight, the transmittance of the dispersion to a visible light of a wavelength of 660 nm is not less than 40%;
(8) the composite cellulose dispersion according to (7) above, wherein the transmittance is not less than 80%;
(9) the composite cellulose dispersion according to (7) or (8) above, wherein the functional additive is selected from the group consisting of a polyhydric alcohol, a water-soluble polysaccharide, a water-soluble polymer and a water-soluble perfume;
(10) a process for producing a cellulose dispersion, which comprises reprecipitating a cellulose solution obtained by dissolving a cellulose in an aqueous inorganic acid solution in water or a coagulating agent containing not less than 50% by weight of water to prepare a cellulose suspension, subjecting the cellulose in the suspension to acid hydrolysis treatment, and subsequently removing the acid from the suspension;
(11) a process for producing a cellulose dispersion, which comprises reprecipitating a cellulose solution obtained by dissolving a cellulose in an aqueous inorganic acid solution in water or a coagulating agent containing not less than 50% by weight of water to prepare a cellulose suspension, subsequently preparing a dehydrated cake of the cellulose from the suspension, thereafter introducing the dehydrated cake into water at not less than 50xc2x0 C. to subject the cellulose to acid hydrolysis treatment and then removing the acid from the suspension;
(12) the process for producing a cellulose dispersion according to (10) or (11) above, which comprises, after the step of removing the acid from the suspension, further subjecting the suspension to high-power grinding treatment;
(13) the process for producing a cellulose dispersion according to (10), (11) or (12) above, wherein the inorganic acid is sulfuric acid;
(14) the process for producing a cellulose dispersion according to (10), (11) or (12) above, wherein the removal of the acid from the suspension is effected by adjusting the pH of the suspension to not less than 2;
(15) the process for producing a cellulose dispersion according to (10), (11) or (12) above, wherein the water in the cellulose dispersion obtained by removing the acid is replaced with a water-soluble organic solvent;
(16) the process for producing a cellulose dispersion according to (10), (11) or (12) above, wherein a functional additive is added to the cellulose dispersion obtained by removing the acid;
(17) the process for producing a cellulose dispersion according to (16) above, wherein after the step of adding the above functional additive, the dispersion is further subjected to high-power grinding treatment;
(18) a cellulose particulate which is composed of a cellulose having a fraction of cellulose I type crystal component of not more than 0.1 and a fraction of cellulose II type crystal component of not more than 0.4 and which has an average particle diameter of 0.2 to 20 xcexcm, a ratio of long diameter (L) to short diameter (D) as observed through a scanning electron microscope (L/D) of not more than 1.2 and a coefficient of aggregation of 1.0 to 3.0, provided that the coefficient of aggregation=(average particle diameter calculated from the volume reduced distribution obtained by a laser diffraction-scattering type particle size distribution measuring apparatus)/(average particle diameter of volume reduction by a scanning electron microscope);
(19) the cellulose particulate according to (18) above, wherein the average degree of polymerization (DP) is not more than 100 and L/D value after pressing at a pressure of 100 kgf/cm2 is not more than 1.2;
(20) a cellulose composite particulate which is a particulate in which at least one functional additive is dispersed in the cellulose particulate according to (18) or (19) above and which particulate has an average particle diameter of 0.2 to 20 xcexcm, a ratio of long diameter (L) to short diameter (D) as observed by a scanning electron microscope (L/D) is not more than 1.2 and a coefficient of aggregation of 1.3 to 3.0;
(21) the cellulose composite particulate according to (20) above, wherein the functional additive is at least one member selected from the group consisting of an oil type compound, a water-soluble polymer, a humectant, a surface active agent, a metal oxide, an ultraviolet screener, an inorganic salt, a metal powder, a gum, a dye, a pigment, carbon black, a silica compound, a latex, an emulsion, an amino acid, a perfume and an antiseptic agent;
(22) a process for producing a cellulose composite particulate which comprises subjecting the cellulose dispersion according to (1), (2), (3) or (4) above to granulation and drying by a spray drying method,
(23) a process for producing a cellulose composite particulate, which comprises mixing the cellulose dispersion according to (1), (2), (3) or (4) above with at least one functional additive and then subjecting the resulting mixed liquid to granulation and drying by a spray drying method;
(24) a cellulose coating film obtained by coating the cellulose dispersion according to (1), (2), (3) or (4) above on the surface of an inorganic material substrate, a plastic substrate or a paper and then drying the same; and
(25) a cellulose composite coating film obtained by coating the composite cellulose dispersion according to (7), (8) or (9) above on the surface of an inorganic material substrate, a plastic substrate or a paper and then drying the same.